A power shear, such as described in commonly owned patent application Ser. No. 072,627, now U.S. Pat. No. 4,267,758 and Ser. No. 072,628, now U.S. Pat. No. 4,267,756 both filed Sept. 5, 1979, has a pair of relatively fixed, spaced-apart, and parallel frame plates that flank a work plate that is displaceable relative to the frame plates in a direction parallel to these frame plates. This work plate acts as a slide and serves to sever a steel member that is positioned passing through aligned apertures in the frame plates, the work plate therefore acting as a blade.
A drive plate is connected to this work plate and in turn is connected to a hydraulic actuator that effects the above-given displacements. The work plate and drive plate are separate from each other so that the work plate can be replaced if necessary or changed to handle different workpieces.
Normally the work plate or drive plate is also connected to several other tools, so that a single shear can be used not only for cutting profile steel members, but also for punching or shearing flat members. The term "shear" is here intended to cover all such machines.
In German published Patent application No. 2,150,542 such a machine is shown wherein the work plate is thinner than the drive plate. The work plate is formed adjacent its end edge that engages the end edge of the drive plate with grooves, and the drive plate is of the same relatively small thickness at these grooves as the work plate. Springs engage between these grooves to hold the two members together. In order to separate the work and drive plates it is necessary to displace the work plate perpendicularly to its normal direction of displacement relative to the drive plate. This motion requires that the machine be set up so as to be openable at one side, and substantially complicates the structure of the machine. In addition force transmission between the end edge of the drive plate carried by the actuator and the work plate is not ideal, as the reduced thicknesses at these end edges occasionally are inadequate so that mushrooming of the end edges with time is possible. Indeed the central plane of the drive plate often lies well to the side of the central plane of the work plate, in fact sometimes it is offset altogether from the work plate.